On Fri, Aug 12, 2016 at 12:57 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Robert Haas <robertmh...@gmail.com> writes:
>> On Fri, Aug 12, 2016 at 9:40 AM, Greg Stark <st...@mit.edu> wrote:
>>> I wonder whether it's really worth keeping postfix operators. They
>>> seem to keep causing these kinds of headaches and I wonder how much
>>> the grammar tables would be simplified by removing them.
>> I've wondered the same thing at other times.  The only postfix
>> operator we ship in core is numeric_fac, and frankly it doesn't seem
>> worth it just for that.  If we decided that factorial(n) was adequate
>> notation for that case, and that we didn't care about any hypothetical
>> user-defined postfix operators either, I think we simplify or improve
>> a number of things.
> [ quick experiment... ]  Simply removing the two postfix-operator
> productions produces no meaningful savings (~0.5%), which is unsurprising
> because after all they're just two more productions to Bison.  It's
> possible we could save more by simplifying the existing hacks elsewhere
> in the grammar that were needed to work around ambiguities induced by
> postfix operators.  But that would take quite a bit of actual work,
> and I suspect we'd end up with a similar result that the tables don't
> actually get much smaller.  You could argue for this on the grounds of
> some reduced intellectual complexity in gram.y, and more forcefully on
> the grounds of removing user surprise in cases like Jim's (especially if
> you can find some other cases where it matters).  But I doubt that we'd
> get any kind of noticeable run-time or code-size win.

Half a percent for two productions is not bad, but I think the real
win would be in removing ambiguity from the grammar.  We get periodic
complaints about the fact that things like "SELECT 3 cache" don't work
because cache is an unreserved keyword, and postfix operators are one
of the reasons why we can't do better:

             * We support omitting AS only for column labels that aren't
             * any known keyword.  There is an ambiguity against postfix
             * operators: is "a ! b" an infix expression, or a postfix
             * expression and a column label?  We prefer to resolve this
             * as an infix expression, which we accomplish by assigning
             * IDENT a precedence higher than POSTFIXOP.

I think I experimented with this a while ago and found that even after
removing postfix operators there was at least one other grammar
problem that prevented us from accepting ColLabel there.  I gave up
and didn't dig further, but maybe we should.  I sort of like the
elegance of supporting user-defied prefix and postfix operators, but
in practice this is a not-especially-infrequent problem for people
migrating from other databases, a consideration that might be judged
to outweigh that elegance.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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